Persistent organic pollutants, POPs for short, are a class of compounds that may be seriously harmful to the environment and human health. They are characterized mainly by toxicity, nondegradable, mobility, bioaccumulation and bioconcentration. POPs are harmful to humans and animals even at a very low exposure extent. POPs pollution has become a global environmental problem.
Incineration is an important method for wastes' disposal. Fly ashes from the wastes' (including domestic wastes, industrial wastes and medical wastes) incineration generally contain a relatively higher concentration of heavy metals and a certain concentration of dioxins. With the wide application of incineration technology for treating the municipal solid wastes, the amount of fly ash generated from the incinerator is increasing. Dioxins are a class of colorless, odorless, fat-soluble substances with serious toxicity. They are two major kinds of organic compounds having very similar structures and properties and including a number of congeners, whose chemical name are respectively called polychlorinated dibenzo-on-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), generally named as PCDD/Fs. Their families of total 210 isomers are important species of persistent organic pollutants (POPs), which exist in the incineration fly ashes and contaminated soils, and they are difficult to be decomposed at ambient temperatures.
In addition, the organochlorine pesticide pollution becomes increasingly serious in China. Organochlorine pesticides (OCPs), mainly including Hexachlorocyclohexane (HCH), dichlorodiphenyltrichloroethane (DDT) and Hexachlorobenzene (HCB), are all made by a chlorination of benzene as raw material. All of them belong to the categories of persistent organic pollutants (POPs). After used, the residual OCPs in soil will be transferred to the water and contaminate the drinking water, or even be transferred to the replanted crops. Pollutants which are similar to the above mentioned pollutants also include Polychlorinated biphenyls (PCBs), also known as chlorinated biphenyl, formed by the linkage of two benzene rings and the substitution of one to ten chlorine atoms for carbon atoms of the benzene rings. There are 209 kinds of isomers in their family, wherein, 12 kinds of coplanar PCBs are toxic. The World Health Organization stipulates the toxic equivalent factor for these 12 kinds of coplanar PCBs, similar to dioxins, so also called dioxin-like PCBs. They have been used extensively in electric devices such as capacitors and transformers as insulating oil due to their chemical stability, favorable thermal conductivity and good insulativity. They were produced in China and many other countries in the world and have been prohibited from the 1970s. However, it is estimated that the total amount of PCBs that exist in ocean, soil and atmosphere in the world reaches more than 25-30 million tons with an extensive pollution since they are difficult to be decomposed.
PBBs and PBDEs are similar to PCBs in properties such as structure, bioconcentration, toxicity and so on, which are the most widely used brominated flame retardants in industry, such as furnitures, textiles, chemicals, electronics, etc. Since they are additive flame retardants without being bound with chemical bonds and would release into the environment by way of volatilization, effusion and the like, they lead to the pollution of air, water, soil and biosphere. Tetrabromodiphenyl ether, pentabromodiphenyl ether, hexabromodiphenyl ether and heptabromodiphenyl ether are listed officially in Stockholm Convention by United Nations Environment Programme in May, 2009. The chemicals used as flame retardants are hexabromo-, octabromo-, nonabromo- and decabromo-compounds. The concentrations of PBBs and PBDEs in the tested environmental samples are higher than PCBs' and they are still used in large quantities, which makes it increase continuously. Therefore, PBBs and PBDEs, with tumorigenic and carcinogenic properties as dioxins, have become one class of the important pollutants. There are still no economical and reliable degradation methods in the prior art.
The current methods for treating the persistent organic pollutants mainly include high temperature processing, supercritical fluid extraction (SCFE), supercritical water oxidation (SCWO), degradation by ultraviolet light, biodegradation, the technique of reductive dechlorination by zero-valent iron, the moderate and low temperature heat reduction processing, and catalytic reduction etc. These methods all have some defects or limitations. For example, the high temperature processing, including melting-vitrification and sintering, can destroy dioxins and prevent their re-synthesis under well-controlled conditions, but the energy consumption is very high and the treatment for the secondary dust and smoke is needed, whereby the cost of the treatment is high and it is difficult for popularizing and implementing.
The relatively effective method for treating the persistent organic pollutants is supercritical water oxidation. For example, CN Patent ZL200510075433.4 disclosed a method of supercritical water oxidation, which is quite effective for the treatment of waste solutions containing toxic organics. Organic compounds, oxygen and water form a single homogeneous phase under the supercritical reaction conditions (Tc≧373.95° C., Pc≧20.64 bar), allowing the oxidation reaction to be quickly performed to effectively destroy organics such as PCDD/Fs. However, the cost and maintenance fee of the reactor is very high when this technique is used as meeting the requirements of engineering design and operation.
Supercritical water has a high ability of dissolving organics, but a low ability of dissolving inorganics. The pharmaceutical in inorganic particles cannot react well in supercritical liquid, unless obtaining a very good dispersion. The other problem in supercritical conditions includes, for example, dioxin-like pollutants contained in fly ash can be oxidized under the supercritical conditions, but the salt in fly ash may precipitate and scale on the wall surface, thus arousing difficulties in the maintenance of the reactor. In order to exert the ability of water for dissolving and destroying organics at high temperature, but to avoid strict requirements for reaction condition or equipment, researchers give an idea that dioxin-like substances are decomposed under subcritical and even high-temperature hydrothermal conditions. The higher the temperature of water is, the greater the ion activity product thereof is, and the higher the ability of oxidation or decomposing to organics is. Therefore the decomposing and oxidation to toxic organics such as dioxins under high-temperature hydrothermal conditions or subcritical conditions are not as good as that under supercritical conditions. High efficiency of decomposing or oxidizing organic pollutants is highly needed under high-temperature hydrothermal conditions or subcritical conditions. CN Patent 200710040771.3 disclosed a method of hydrothermal treatment of waste incineration fly ash, specifically including that the hydrazine substance is added to facilitate the decomposition of dioxin in the hydrothermal reaction for treating dioxins contained in waste incineration fly ash, and including the technique of anti-corrosion of wall surface. This method facilitates the decomposition of dioxin under subcritical conditions, but pre-decomposition of hydrazine is needed to be prevented when the method is implemented, and a certain rate of cooling is needed.
In addition, the technique of reductive de-chlorination by zero-valent iron is in the prior art, which can be implemented not only at room temperature (Chen Shaojin, Liang Hesheng, Experimental Study of the Reductive De-chlorination of PCBs in Polluted Soils by Zero Valent Iron, Ecology and Environmental Sciences 2009, 18(1): 193-196), but also under hydrothermal conditions (YAK H K, WENCLAWIAK B W, CHENG I F, et al. Reductive de-chlorination of polychlorinated biphenyls by zero-valent iron in subcritical water, Environmental Science and Technology, 1999, 33: 1307-1310; Nikolay Kluyev, Andrei Cheleptchikov et al., Reductive de-chlorination of polychlorinated dibenzo-p-dioxins by zero valent iron in sub-critical water, Chemosphere 2002, 46: 1293-1296). However, the zero-valent iron (Fe0) of solid state in the mixture of water/particles cannot get good contact with particles, the dissolved PCDD/Fs, PCBs and other pollutants, thus the improvement of efficiency is limited. Moreover, hydrogen is often generated when Fe0 exists in the subcritical water, bringing a risk of increased pressure in the reaction vessel and a potential danger in operation. In addition, the technique of reductive de-chlorination by zero-valent iron is hardly to be used for the de-chlorination of PCBs and dioxins with less chlorine substitutions, and only can be used for the de-chlorination of PCBs and dioxins with more chlorine substitutions. De-chlorination reaction is carried out on the surface of zero-valent iron particles. When particles (such as multi-hole particles as active carbon in incineration fly ash) have a high ability of absorption for PCBs and dioxins, the absorbed dioxins and PCBs, PBBs, PBDEs and halogenated polycyclic aromatic hydrocarbons have no chance to react with zero-valent iron.
In order to treat the organic pollutants in solid particles more economically, safely and efficiently, those skilled in the art devote themselves to developing a process for eliminating or reducing the organic pollutants like dioxins, organochlorine pesticide and PCBs, PBBs and PBDEs contained in solid particles.